1. Field of the Invention
The present invention relates to a camshaft for use in internal combustion engines, and more particularly, to a cam design and method of assembly.
2. Description of the Related Art
Conventional camshafts used to control valve motion in internal combustion engines include a shaft having axially spaced cams, which project outward from the surface of the shaft. The shaft and cams can be machined from a single casting or forging, but are usually assembled from separate parts. Each cam is mechanically coupled to one of the engine valves so that rotation of the shaft results in valve movement. In addition to the cams, conventional camshafts include journals, fittings, sensors, and balancing masses mounted to the shaft.
FIG. 1 and FIG. 2 show, respectively, a side view of a portion of a conventional camshaft 10, and a cross-sectional view of the camshaft 10 through section line 2. The camshaft 10 includes a tubular shaft 12 having inner 14 and outer surfaces 16 and an annular or ring-type cam 18 mounted on the outer surface 16 of the shaft 12. As shown in FIG. 2, the cam 18 includes a lobe boss portion 20 and a ring portion 22 having respective inner 24, 26 and outer surfaces 28, 30. The inner surface 24 of the lobe boss 20 and the inner surface 26 of the ring portion 22 of the cam 18 define a continuous mounting surface for joining the cam 18 to the outer surface 16 of the shaft 12. During operation of the camshaft 10, the outer surface 28 of the lobe boss 20 generates the desired valve lift, while the outer surface 30 of the ring portion 22 defines a base circle, which provides zero-valve lift. To assemble the conventional camshaft 10, each of the cams 18 are positioned over an end of the shaft 32, translated to a pre-defined axial position, and attached or joined to the outer surface 16 of the shaft 12.
Although generally satisfactory, conventional camshaft designs can be improved. For example, to set the relative angular position of the cams around the periphery of the shaft (timing angle), conventional camshafts typically employ a ring-type cam having polygonal or spline mounting surfaces that interlock with matching surfaces on the outer surface of the shaft. As a result, any necessary adjustments in lift or timingxe2x80x94e.g., changes in the relative angular position of the camsxe2x80x94require costly changes to the shaft and cams. In addition, to reduce overall camshaft weight and cost, recent cam designs have sought to minimize wall thickness of the ring portion of the cam and the shaft. However, insufficient wall thickness may result in undesirable thermal distortion, severe cold working or thinning during assembly, and marginal mechanical performance. Furthermore, ring-type cams often require preprocessing of the shaft, such as forming and precision machining which increases costs and process variability. The wall thickness of the ring portion of the cam also limits the outer diameter of the shaft and journal, which may result in increased journal dynamic bearing loading and decreased camshaft service life.
In many cases, use of ring-type cams also requires complex joining or attachment methods, including shrinkage joining and hydroforming. Although used successfully to assemble camshafts, both techniques present difficulties. For instance, when using shrinkage techniques only a small percentage of the cam mounting surface contacts the outer surface of the shaft. As a result, shrinkage techniques require precision ground components that must be carefully positioned to prevent attachment failures. Although hydroforming may work well on thin wall cams subject to low stress, the method is impractical for relatively high stress loadings of most current automotive and diesel engines. In addition, hydroforming uses large and expensive equipment and tooling, and requires lengthy development time since iterative testing is often necessary to optimize material flow and strength characteristics.
Other complex methods of attachment, such as ballizing, sinter brazing, and liquidous-type expansion joining, also present difficulties. For example, ballizing is an expansion technique requiring the use of highly controlled tube wall and outside shaft geometry as well as an expensive die arrangement for assembly. Common problems with ballizing include part distortion and inconsistent material properties. Sinter brazing uses a filler agent, which adds expense and material coverage problems. It also requires the use of a high temperature furnace and lengthy heating and cooling cycles to process the camshaft assembly, which may lead to thermal distortion of the camshaft. Like shrinkage joining and ballizing, sinter brazing requires precision components to optimize joining characteristics. Finally, liquidous-type expansion techniques employ concentric tubes and a liquid crystalline polyester resin, which is injected into an annular gap between the tubes. Since multiple tubes are used, the method is costly.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.
One aspect of the present invention provides a camshaft assembly for transmitting and controlling valve motion in an internal combustion engine. The camshaft assembly includes a shaft having an outer surface and a longitudinal axis, and a cam that is mounted on the shaft. The cam includes a lobe boss portion having a pair of side walls and a transverse surface. The transverse surface of the lobe boss portion of the cam bridges the pair of side walls and defines a cam profile that provides the requisite valve lift and valve velocity during operation. The cam also includes a base portion that provides a surface for joining the cam to the shaft at a predetermined position along the longitudinal axis of the shaft. In contrast to ring-type cams, the base portion or the mounting surface of the cam does not circumscribe the outer surface of the shaft, but instead extends only part way around the circumference or periphery of the shaft. This allows for radial mounting of the cams at virtually any relative angular displacement or timing angle. Because the cams of the present invention lack a ring portion, the cam width adjacent to the base portion can be made narrower, which allows for greater flexibility in the design of the cam profile shape and the resulting cam lift curves.
Another aspect of the present invention provides a method of assembling a camshaft. The method includes providing components that make up the camshaft, such as a shaft and cams, and radially mounting at least one of the cams on the shaft. The mounting step includes positioning the cam at a pre-mounting location that is spaced away from an outer surface of the shaft and located between ends of the shaft, and placing the cam on the outer surface of the shaft at a mounting angle of about 90xc2x0. A mounting angle of 90xc2x0 corresponds to placing the cam on the shaft normal to a plane containing a longitudinal axis of the shaft. In contrast to assembling ring-type cams, which require complicated joining or attachment methods, radial mounting can use simpler joining methods such as capacitance discharge welding.